Abstract
The big‐scale sand smelt Atherina boyeri lives in fresh water, brackish water and sea water of the western Atlantic Ocean and Mediterranean Sea. Previous studies concerning distribution, biometric characters and genetic molecular markers have suggested the possible existence of two or even three different groups or species of sand smelt, one ‘lagoon’ type and one (or two – punctuated and non‐punctuated on the flanks) ‘marine’ type. In this study, the presence and the localization of an insertion was described, c. 200 bp in length, in the mtDNA of the lagoon and marine punctuated specimens of A. boyeri and its absence in the marine non‐punctuated specimens, as well as in other two congeneric species, Atherina hepsetus and Atherina presbyter , and in the atheriniform Menidia menidia. The intergenic spacer is located between the tRNA Glu and cytochrome b (cyt b ) genes and shares a c. 50% sequence similarity with cyt b . The distribution and the features of the intergenic spacer suggest that it might have originated from an event of gene duplication, which involved the cyt b gene (or, more likely, a part of it) and which took place in the common ancestor of the lagoon and the marine punctuated specimens. The data obtained therefore support the hypothesis of the existence of three cryptic and, or sibling species within the A. boyeri taxon and provide a genetic molecular marker to distinguish them.